At present, global environmental issues are gaining increasingly more attention. One of the important and urgent tasks is the reduction of nitrogen oxides represented by NOx. In methods for reducing NOx, techniques for suppressing NOx emission are important, and include exhaust gas recirculation, lean combustion, thick and thin combustion, multi-stage combustion, and the like, which are widely used from the industrial to the customer market. Low-NOx combustors to which such a technique is applied have promoted the reduction of NOx to some degree. However, more effective methods for reducing NOx have been further required.
One of the methods for reducing NOx that has hitherto been studied and developed is a method which involves cyclically changing the flow rate of fuel, or air or the like serving as an oxidant to perform one kind of thick and thin combustion temporally controlled (hereinafter referred to as a “forced oscillating combustion”). This kind of method has been proposed (see Patent Literatures 1 to 6).
In the method, the flow rate of supply of one of a fuel fluid and an oxidant fluid, or both the fuel fluid and the oxidant fluid is changed to vary an oxygen ratio of combustion flame (that is, a value obtained by dividing an amount of supply of oxygen by a theoretically required oxygen amount) thereby alternately performing fuel-rich combustion and fuel-lean combustion. As a result, the method achieves the reduction of NOx in the combustion gas.
Patent Literature 7 discloses a method for reducing nitrogen oxides which involves using oscillating combustion, that is, so-called forced oscillating combustion under a high concentration of pure oxygen as an oxidant, and also a device for performing the method.
In general, a heating furnace and a melting furnace are provided with a plurality of burners. In applying the forced oscillating combustion to each burner, combustion conditions and oscillation cycles should be appropriately controlled to obtain a great effect of NOx reduction.